Valves are commonly used to control the flow characteristics of a fluid. A conventional valve includes a valve body defining an inlet, an outlet, and a fluid flow path extending between the inlet and the outlet. A throttling element, such as a plug, is disposed in the path to control fluid flow. A stem connects the valve plug to an actuator, which moves the valve plug into and out of the flow path. A conventional valve may include a cage extending across the fluid flow path and having orifices or apertures formed therein. The plug has an outer surface sized to closely fit inside an inner cylindrical wall of the cage. As a result, the plug is slidable inside the cage between a closed position, in which the plug obstructs the orifices, and an open position, in which the plug is clear of at least a portion of the orifices. The plug can also be positioned between fully open and closed positions to control the flow rate of fluid through the cage. In a fully open position, the downstream or proximal end of the valve plug may be received in a retainer, which may be part of the cage assembly, the bonnet or the valve body.
To effect a seal between the plug and the cage, a seat ring can be disposed at the upstream or inlet end of the cage. The seat ring provides a seating surface for a seat disposed at a distal or upstream end of the valve plug. The combination of the seat on the valve plug and the seating surface on the seat ring define what is known as the trim set for the valve.
Similarly, to effect a seal between the proximal or upstream end of the valve plug and the cage retainer, the proximal end of the valve plug also typically includes some sort of seat or surface for engaging the cage retainer.
For valves used in high temperature operations, e.g., the transfer of super heated steam, the seat of the valve plug and the seat ring disposed at the upstream end of the cage may be fabricated from metal because of the inability of polymeric seals to withstand such high temperatures. Further, the valve body, the cage and the plug body or spacer tube of the plug are also typically fabricated from metal, such as alloy steels and stainless steels.
One problem associated with such high temperature valves is related to the thermal expansion of the various parts when the valves are used at high operational temperatures. Typically, the material used to construct the valve body, such as an alloy steel, will not have the same rate of thermal expansion as that of the trim parts, i.e., the plug body, seat rings, cage and seat ring, which may be fabricated from different alloy steels or stainless steels. Thus, as a metallic valve assembly operates at high temperatures, the critical dimensions of the various components will change and the valve may not open and seal properly or throttle properly. As a result, the valve does not function well and the valve stem may be prone to premature breakage and/or other failures.
One solution to this thermal expansion problem would be to fabricate all of the components from the same material. However, for larger valves operating at over 1000° F., such a strategy is not practical. For corrosion inhibition, reduced maintenance and for fabrication issues, the cage is preferably fabricated from a stainless steel. For cost considerations, the large valve body is preferably fabricated from a cheaper, alloy steel.
Depending upon the particular design, at high temperatures, the axial expansion of a valve assembly is dominated by the valve body and the radial expansion of the valve assembly is dominated by the cage, the plug body and the cage retainer, if a retainer structure is employed. Therefore, there is a need for an improved valve assembly whereby the valve plug expands axially in the same manner as the valve body and radially in the same manner as the cage and the retainer.